CRTs are used in a wide variety of applications ranging from the conventional color television to high resolution computer monitors and very high resolution medical applications. In a color CRT, a plurality of electron beams are horizontally swept across the CRT's display screen in a raster-like manner, while in a monochrome CRT a single electron beam is displaced over the CRT display screen. Two of the more important operating criteria for both color and monochrome CRT displays are video image resolution and brightness. Video image brightness is particularly important in a television receiver CRT because the viewer is typically positioned a substantial distance from the CRT and because of the wide range of video image shades and, in the case of a color CRT, hues. Video image resolution is particularly important in a computer monitor because of the small size of the characters and graphics and close spacing between video image elements. Unfortunately, these two operating criteria are interrelated such that improvement in one performance parameter generally has an adverse effect on the other.
One approach to providing acceptable image brightness involving the use of higher beam currents employs a dispenser cathode which affords high electron emission densities. Another approach to increasing beam current and thus image brightness involves the use of larger electron guns to obtain a small spot size necessitating a larger CRT envelope neck size. Increasing the size of the CRT envelope is contrary to current trends which seek to reduce the non-display screen portions of the CRT.
The present invention addresses the aforementioned problems encountered in the prior art by providing a multi-mode, hybrid-type CRT, and electron gun therefor, which can be used equally as well in a conventional television receiver in providing a high level of video image brightness or in a computer monitor for displaying high resolution graphics and alphanumeric character images. The inventive multi-mode, hybrid-type CRT and electron gun therefor directs a small diameter beam, or beams in the case of a color CRT, on the display screen for high video image resolution, or a larger diameter, higher current beam, or beams, for increased image brightness in a second mode of operation, where the two modes are selectable by a viewer.